[Observability] Add Network Topology plugin docs

[mdbirnstiehl]

Summary

This PR brings the content from elastic/observability-docs#4981 to 9.4.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

Tool(s) and model(s) used: Claude Code

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🔍 Preview links for changed docs

• solutions/observability/infra-and-hosts/network-topology.md
• solutions/observability/infra-and-hosts/network-topology/field-reference.md
• solutions/observability/infra-and-hosts/network-topology/monitor-network-devices.md

[github-actions]

Vale Linting Results

Summary: 3 warnings, 1 suggestion found

⚠️ Warnings (3)

File	Line	Rule	Message
solutions/observability/infra-and-hosts/network-topology/field-reference.md	67	Elastic.Spelling	'Gbps' is a possible misspelling.
solutions/observability/infra-and-hosts/network-topology/field-reference.md	135	Elastic.Spelling	'adjacencies' is a possible misspelling.
solutions/observability/infra-and-hosts/network-topology/monitor-network-devices.md	126	Elastic.Spelling	'adjacencies' is a possible misspelling.

💡 Suggestions (1)

File	Line	Rule	Message
solutions/observability/infra-and-hosts/network-topology/field-reference.md	52	Elastic.Semicolons	Use semicolons judiciously.

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